To fall or sink heavily; collapse: She slumped, exhausted, onto the sofa. To droop, as in sitting or standing; slouch. To decline suddenly; fall off: Business slumped after the holidays. To perform poorly or inadequately: The team has been slumping for a month. To sink or settle, as into mud or slush. To slide down or spread out thickly, as mud or fresh concrete. The act or an instance of slumping. A drooping or slouching posture: read defeat in the slump of his shoulders. A sudden falling off or decline, as in activity, prices, or business: a stock market slump; a slump in farm prices. An extended period of poor performance, especially in a sport or competitive activity: a slump in a batting average. See grunt.